U.S. Pat. No. 4,467,770 discloses a method and apparatus for feedback controlling the air-fuel ratio in an internal combustion engine at the stoichiometric air-fuel ratio by using an oxygen sensor having a stepwise output characteristic at the stoichiometric air-fuel ratio wherein an integration compensation factor calculated depending upon detected air-fuel signal and a learning compensation factor determined by the integration compensation factor are used for the feedback control, however the feedback control of the air-fuel ratio is effected at the stoichiometric air-fuel ratio for the engine as a whole without regarding the air-fuel ratios of respective exhaust gas from the respective cylinders.
Japanese Patent Application Laid Open No. 59-23046(1984) discloses an air-fuel ratio feedback controlling apparatus for an internal combustion engine with a plurality of cylinders wherein an air-fuel ratio sensor having a linear output characteristic is used for detecting the respective air-fuel ratio in the exhaust gas from the respective cylinders and the air-fuel ratios of the respective cylinders are feedback controlled, however no learning operation is effected in the apparatus.